US Air Force Gen. Philip Breedlove has threatened Russia with military response should Moscow secretly send forces with or without military uniform into Ukraine.

“If NATO sees foreign forces infiltrating its sovereign territory, and if we can prove it comes from an aggressor nation then that’s Article five,” Gen. Breedlove said.

“That means a military response to the actions of the aggressor. Therefore, we have to expand the internal national capabilities to identify such a move and to deal with it.”

The top military commander of NATO and commander of US European Command issued the warning in an interview with the German newspaper Die Welt.

He also noted that NATO must take into account Russian actions in Ukraine as it prepares for future threats.

Washington accuses Moscow of arming pro-Russian forces in eastern Ukraine and dispatching Russian military personnel to the country. However, the Kremlin denied the allegation.

Gen. Breedlove said NATO must be ready to counter such unconventional tactics.

“The most important thing is that NATO nations are prepared for the so-called little green men,” he said. “We’ve seen this in eastern Ukraine … And there is a danger that it also could happen in other East European states. Therefore, we have to train the police and army there in such a way that they can deal with this kind of challenge.”

On Friday, the US warned Russia over what it calls “extremely dangerous and provocative” efforts to destabilize Ukraine.

The US National Security Council warned of an escalation in the crisis after Ukrainian President Petro Poroshenko claimed that his military had destroyed a Russian military convoy that had ventured into Ukraine. Moscow has dismissed the allegation as “fantasies.”